This invention relates to rechargeable batteries and battery packs and, in a particular embodiment, to smart batteries and battery packs (batteries/packs). The subject invention pertains to improving the accuracy of battery/pack capacity, remaining capacity, and remaining run time determinations. Advantageously, this invention can utilize a dynamic end of charge voltage to enhance the performance of rechargeable batteries/packs throughout the battery/pack life and over a wide variety of operating conditions and environments. This invention can enable battery/pack users to get more useable capacity out of each charge of the battery/pack and to more accurately track the battery/pack capacity.
Rechargeable batteries/packs are often used in applications, for example laptop computers, where battery/pack supplied energy is necessary to maintain certain information related to the user's application. In these situations, when the battery no longer can supply the ongoing energy requirement, some or all of this information can be lost. Accordingly, device designers and manufactures have incorporated techniques to alert a user and/or signal a host device as to the remaining capacity in the battery/pack, for example to initiate save-to-disk routines for laptop computers to avoid loss of information. By alerting a user when the battery/pack is approaching a point where the battery/pack has only enough capacity remaining to perform the save routine, a user can perform the save routine in time to avoid loss of valuable information. It is the goal to initiate these save routines at a point in the discharge cycle where enough capacity remains to perform the save routine with adequate margins for error, while minimizing excess unused capacity in the battery/pack.
Additionally, other events may be triggered when the battery/pack reaches certain output voltage levels, wherein the remaining charge is presumed to be correlated to these certain output voltage levels. For example, fuel gauge operations, shut down, and other remaining capacity communications with a host device or user can be triggered based on the output voltage level of the battery/pack.
Typically, a fixed End of Discharge Voltage (EODV) is used in battery/pack operating and charging algorithms, where the EODV is a voltage output of the battery/pack which, when reached, indicates that essentially all of the usable capacity of the battery/pack, over and above the capacity needed for any necessary terminal function, has been removed. Accordingly, for example in the context of a smart battery/pack, once this fixed EODV is reached an indication would be given to the host device or user to shutdown and recharge the battery/pack. Additionally, this fixed EODV often functions as the endpoint for determining the capacity of a battery/pack such that the capacity is defined to be essentially zero, or fixed finite amount, at the EODV. Accordingly, the accuracy of this EODV is very important to the performance of the battery/pack.
Due to internal resistance (IR) drops, for example due to the cell and interconnects, a fixed EODV is often inaccurate for these purposes. These IR drops from contact resistance, cell can resistance, wire resistance, trace resistance, electrode resistance, electrode/electrolyte interface resistance, electrolyte resistance, and protection circuitry can lead to an incorrect determination of the actual potential of the electrode of the battery/pack. In particular, at high discharge rates these IR drops can alter a battery/pack's discharge profile as a function of discharge rate and, due to the use of a fixed EODV, may lead to the determination that the battery/pack has delivered all the available capacity even when more capacity may still be available from the battery/pack. Furthermore, these IR drops can reduce the accuracy, or repeatability, of the fuel gauge at various discharge rates, for example when a fixed EODV is used to define zero, or near zero, capacity.
In addition, a fixed EODV can lead to inaccurate capacity determination as the number of times the battery/pack has been charged and discharged increases. For rechargeable batteries/packs, the voltage profile as it relates to capacity changes as a function of cycle life. Specifically, for certain batteries/packs, as the number of charging cycles increases the fixed EODV is reached at a point where more capacity is actually available than when the same fixed EODV is reached when the battery/pack had undergone fewer charging cycles.
Furthermore, other factors such as temperature, cell chemistry, form factor, residual capacity, and operating conditions can affect the accuracy of the capacity determinations. Accordingly, there is a need in the art for a method and apparatus to more accurately gauge remaining capacity and more accurately determine the end of discharge voltage for rechargeable batteries/packs.